I found the following in a copy of the "International Correspondence School Handbook, Malleable Casting, Brass Founding, Smithing and Forging". Date about 1900, nice leather bound jobs with excellent line illustrations.
The cutting irons of hand planes, jack planes, moulding planes, etc., were formerly made by welding a tool-steel face to a piece of wrought iron. At present they are very generally made from a fine quality of rolled or cast steel. The blanks are sheared or punched to the proper size, and any necessary machine work is done on them, including grinding the bevel.
The cutting end is the heated to a cherry red by placing it in a lead pot. To ensure heating to just the right distance from the cutting edge the cutting irons are held crossways, in a special pair of tongs, in such a way that when the tongs rest on the edge of the lead pot the cutting irons will extend into the lead the correct distance. Several cutting irons may be heated at one time;
When taken from the lead one at a time they are dropped into a tank of brine, or they are plunged with the tongs, moving the pieces about rapidly until cool, then allowing them to drop to the bottom of the tank. The plane irons drop into a wire basket, by which they can be lifted out when a sufficient number have accumulated.
The temper is drawn between hot-iron plates pressed together by a cam-motion hand press, which not only serves to bring the plates into contact with the plane iron, but also assists in flattening it. The temper is regulated by the length of time the cutting iron is between the plates.
Occasionally, one of the irons is tested for hardness with a file. It is claimed that greater uniformity of temper can be obtained in this manner than is the case when judging by colour".
Compare the tempering process with the control obtained by thermostatically controlled furnaces and checking with hardness testing machines.